High-silicon ferritic heat-resistant cast iron having high-temperature strength and high oxidation resistance

ABSTRACT

Disclosed are a ferritic heat-resistant cast iron comprising 5.0˜7.0 weight % of Silicon (Si) to improve oxidation resistance of the cast iron and a method for preparing the same.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2006-0101452, filed on Oct. 18, 2006, the entire disclosure of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a high-silicon ferritic heat-resistant cast iron having excellent high-temperature strength and/or oxidation resistance. More particularly, it relates to a high-silicon spheroidal graphite cast iron having excellent high-temperature crack resistance and/or oxidation resistance, which can be suitably applied to various parts of exhaust systems of a vehicle that undergo repeated process of heating and cooling, such as an exhaust manifold and a turbine housing.

2. Description

In general, FCD-HS, SiMo cast irons have been used as materials for exhaust manifolds of various kinds of vehicles. Such materials are made by adding Si, Cr, Mo, etc. to an existing spheroidal graphite cast iron to improve the oxidation resistance and the physicochemical properties at a high temperature. Normally, the temperature range of the exhaust system employing the heat-resistant cast iron is approximately 630° C. to 750° C.

However, with the recent increase in engine power of vehicle, the temperature of exhaust gas has rapidly increased to 890° C. in maximum, which causes some drawbacks in that it significantly burdens the exhaust manifold and it creates catalyst damages due to the high-temperature oxidation scale.

As the exhaust manifold is accompanied with thermal expansion and contraction that expose the exhaust manifold to high and low temperatures repeatedly, one additional drawback caused by the increased exhaust temperature is that the materials for the exhaust manifold can be heat-distorted, which in turn causes surface oxidation and further penetration cracks.

These surface oxidation and penetration crack occur frequently in the region of a port portion or a link portion of the exhaust manifold that is relatively weaker than the other portions, which may result from the fact that the port portion is thin and significantly influenced by the unbalanced metal structure due to an associated cooling.

Recently, high-speed performance of vehicle engines has been improved by raising exhaust temperature and various attempts to raise the exhaust temperature have been made. To achieve a successful result, it is necessary to develop a heat-resistant cast iron material having improved heat distortion resistance and/or oxidation resistance.

To date, spheroidal graphite cast irons contain approximately 4-5% of silicon have been widely used since they have good castability and relatively low manufacturing cost.

Such high-silicon spheroidal graphite commonly contains, for example, 3.5-5.0 weight % of silicon. If the silicon content does not exceed 4.0 weight %, it has no difference in the oxidation resistance compared to conventional graphite cast irons. On the other hand, if it exceeds 4.5 weight %, hard and fragile silicon ferrite is segregated into a matrix structure as an inclusion, thereby deteriorating elongation at room temperature and machinability as well. Accordingly, silicon content of the high-silicon cast iron material has been limited to 4.0-4.5 weight %.

Taking such problems into consideration, a high-nickel austenitic cast iron containing nickel in quantities has been used as a heat-resistant spheroidal graphite cast iron for the exhaust system parts capable of coping with the high temperature of exhaust gas.

However, as the high-nickel spheroidal graphite cast iron comprises about 25-35 weight % of nickel, the manufacturing cost is relatively high. Also, castability and machinability are relatively low.

Alternatively, Japanese Patent Publication No. 1997-087796 discloses a heat-resistant spheroidal graphite cast iron comprising 2.7-3.2 weight % of C, 4.4-5.0 weight % of Si, 0.6 weight % or less of Mn, 0.5-1.0 weight % of Cr, 0.1-1.0 weight % of Ni, 1.0 weight % or less of Mo, 0.1% or less of spheroidizing agent, and the balance of Fe. In addition, Japanese Patent Publication No. 2002-371335 discloses a heat-resistant spherical graphite cast iron for exhaust parts with superior oxidation resistance, comprising 2.5-4.1 weight % of C, 3.4-5.0 weight % of Si, 1.0 weight % or less of Mn, 0.1 weight % or less of P, 0.01 weight % or less of S, 0.02-0.10 weight % of Mg, 0.1-1.5 weight % of Mo, 0.5-2.0 weight % of Ni, and the balance of Fe.

However, there is still a need for a heat-resistant cast iron that can provide higher heat distortion resistance and oxidation resistance than those disclosed in the above patents.

The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.

SUMMARY OF THE INVENTION

In one aspect, the present invention provides a ferritic heat-resistant cast iron comprising 5.0˜7.0 weight % of silicon to improve oxidation resistance of the cast iron.

In a preferred embodiment of the present invention, cast irons may further comprise 0.01˜0.1 weight % of chrome to further improve oxidation resistance.

In another preferred embodiment, cast irons of the present invention may further comprise 0.5˜1.2 weight % of molybdenum to improve high-temperature strength.

In still another preferred embodiment, cast irons of the present invention may further comprise 0.5 weight % or less of vanadium to improve high-temperature strength.

In another aspect, the present invention provides a high-silicon ferritic heat-resistant cast iron having improved high-temperature strength and oxidation resistance comprising: 2.5˜3.8 weight % of carbon (C), 5.0˜7.0 weight % of silicon (Si), 0.2˜0.8 weight % of manganese (Mn), 0.05 weight % or less of phosphorus (P), 0.02 weight % of less of sulfur (S), 0.5˜1.2 weight % of molybdenum (Mo), 0.1 weight % or less of nickel (Ni), 0.5 weight % or less of vanadium (V), 0.01˜0.1 weight % of chrome (Cr), 0.05 weight % or less of antimony (Sb), and the balance of ferrum (Fe).

In still another aspect, the present invention provides a method for preparing a high-silicon ferritic heat-resistant cast iron having high-temperature strength and high oxidation resistance, comprising the steps of: (a) casting an iron alloy material comprising 2.5˜3.8 weight % of C, 5.0˜7.0 weight % of Si, 0.2˜0.8 weight % of Mn, 0.05 weight % or less of P, 0.02 weight % of less of S, 0.5˜1.2 weight % of Mo, 0.1 weight % or less of Ni, 0.5 weight % or less of V, 0.01˜0.1 weight % of Cr, 0.05 weight % or less of Sb, and the balance of Fe; and (b) annealing the cast iron in a manner that the temperature is raised up to 960° C. at a rate of 20° C./min, the raised temperature is kept for 20 minutes and them slowly cooled down to 400° C. at a rate of 20° C./min.

In a further aspect, motor vehicles are provided that comprise a described cast iron.

It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like. The present cast iron will be particularly useful with a wide variety of motor vehicles.

Other aspects of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing results of high-temperature tensile tests for a high-silicon ferritic heat-resistant cast iron according to a preferred embodiment of the present invention; and

FIG. 2 is a graph showing results of high-temperature oxidation tests for a high-silicon ferritic heat-resistant cast iron a preferred embodiment of the present invention.

DETAILED DESCRIPTION

As discussed above, the present invention provides a high-silicon ferritic heat-resistant cast iron having improved high-temperature strength and/or oxidation resistance and a method for preparing the same.

Reference will now be made in detail to the preferred embodiment of the present invention, examples of which are illustrated in the drawings attached hereinafter, wherein like reference numerals refer to like elements throughout. The embodiments are described below so as to explain the present invention by referring to the figures.

A preferred high-silicon spheroidal graphite cast iron material of the present invention has excellent oxidation properties and mechanical properties such as high-temperature tensile strength and thus can be applied suitably to an exhaust manifold of an exhaust system and a turbine housing of a turbo charger.

High-silicon cast iron materials of the present invention may be manufactured by adding Si, Mo, Ni, V and Cr to a spheroidal graphite cast iron having a matrix structure.

For this purpose, high-silicon heat-resistant cast irons of the present invention may preferably be manufactured by comprising 2.5˜3.8 weight % of C, 5.0˜7.0 weight % of Si, 0.2˜0.8 weight % of Mn, 0.05 weight % or less of P, 0.02 weight % of less of S, 0.5˜1.2 weight % of Mo, 0.1 weight % or less of Ni, 0.5 weight % or less of V, 0.01˜0.1 weight % of Cr, 0.05 weight % or less of Sb, and the balance of Fe.

The reasons for adding the respective constituents contained in the cast iron material of the present invention and for limiting each the contents to a specific range will be described below.

(1) Carbon (C): 2.5-3.8 Weight %

The carbon content added should be limited to, preferably, 2.5-3.8 weight % in consideration of the decrease in flowability and the primary graphite precipitation.

(2) Silicon (Si): 5.0˜7.0 Weight %

Silicon is an element to be contained for the graphite precipitation and it is desirable that its content added be limited to 5.0˜7.0 weight % in consideration of the ferrite reaction of matrix, the increase in oxidation resistance, the synergistic effect on transformation from ferrite to austenite, the melt flow during casting and the machinability.

(3) Manganese (Mn): 0.2˜0.8 Weight %

As manganese is segregated into eutectic cell boundaries, which lowers the transformation temperature from ferrite to austenite, its content added should be limited to 0.2˜0.8 weight %.

(4) Phosphorus (P): 0.05 Weight % or Less

As phosphorus forms steatite, its content added should be limited to 0.05 weight % or less.

(5) Sulfur (S): 0.02 Weight % of Less

As sulfur is to graphite spheroidization, it is desirable to regulate the content added at 0.02 weight % of less.

(6) Molybdenum (Mo): 0.5˜1.2 Weight %

Molybdenum is added to form precipitated carbides by combining with carbon and to decrease an average thermal expansion coefficient to lower the generation of thermal stress at high temperature, thus enhancing the high-temperature stress. If it is added in quantities, it may increase the amount of intergranular carbides to deteriorate the machinability and the elongation at room temperature. Accordingly, it is desirable to limit the molybdenum content added to 0.5˜1.2 weight %.

(7) Nickel (Ni): 0.1 Weight % or Less

Nickel is added for the purpose of improving the elongation at room temperature and its content added should be limited to 0.1 weight % or less.

(8) Chrome (Cr): 0.01˜0.1 Weight %

Chrome is added as an element for forming chromium oxides and improving the oxidation resistance. Since it may deteriorate the machinability if it is added in quantities, its content added should be limited to 0.01˜0.1 weight %.

(9) Vanadium (V): 0.5 Weight % or Less

Vanadium is added to obtain an effect of improving the high-temperature strength, i.e., an effect of strengthening precipitation by VC, and its content added is limited to 0.5 weight % or less. Addition of more than 0.5 weight % of V would not produce any other effect than what can be obtained with 0.5 weight % or less of V.

(10) Antimony (Sb): 0.05 Weight % or Less

0.05 weight % or less of Sb is added for the purpose of obtaining an effect of lessening the segregation between graphite elements by increasing the graphite nodule count.

As discussed above, in another aspect, the present invention provides a method for preparing a high-silicon ferritic heat-resistant cast iron having high-temperature strength and high oxidation resistance. Preferably, a preferred method for preparing such high-silicon ferritic heat-resistant cast iron may suitably comprise the steps of: (a) casting an iron alloy material comprising 2.5˜3.8 weight % of C, 5.0˜7.0 weight % of Si, 0.2˜0.8 weight % of Mn, 0.05 weight % or less of P, 0.02 weight % of less of S, 0.5˜1.2 weight % of Mo, 0.1 weight % or less of Ni, 0.5 weight % or less of V, 0.01˜0.1 weight % of Cr, 0.05 weight % or less of Sb, and the balance of Fe; and (b) annealing the cast iron in a manner that the temperature is raised up to 960° C. at a rate of 20° C./min, the raised temperature is kept for 20 minutes and them slowly cooled down to 400° C. at a rate of 20° C./min.

In step (a), cast iron material of the present invention can be manufactured by any well-known method to those skilled in the art.

In a preferred embodiment of the present invention, the annealing process of step (b) may suitably conducted in a manner that the temperature is raised up to 96020 C. at a rate of 20° C./min, the raised temperature is kept for 20 minutes and slowly cooled at a rate of 20° C./min down to 400° C.

High-silicon cast irons of the present invention manufactured as described above include vanadium carbides (VC) formed in the matrix by the addition of vanadium (V) and this matrix structure enhances the high-temperature strength.

Furthermore, the added molybdenum (Mo) provides an effect of solid solution strengthening of ferrite to enhance the high-temperature strength.

It would be desirable to generate Fe₂SiO₄ that has a close connection with the interface between oxide layer and matrix as much as possible for improvement of the oxidation resistance. For this purpose, it would be necessary or desirable to add 5.0 weight % or more of Si to the cast iron material.

However, although Si is an element that raises the transformation temperature from ferrite to austenite in addition to the oxidation resistance, the ferrite hardness will be increased to deteriorate the ferrite ductility, if Si content exceeds 4.4 weight %. Accordingly, in the present invention, the cast iron material having high Si content is subjected to the above-described annealing process to ensure the oxidation resistance and prevent the ductility deterioration after casting.

Accordingly, high-silicon cast irons of the present invention can show excellent heat-resistant properties and oxidation resistance and can be suitably applied to exhaust manifolds of high power vehicle engines.

The following examples are presented to illustrate further various aspects of the present invention, but are not intended to limit the scope of the invention in any aspect.

EXAMPLE

To prepare a high-silicon ferritic heat-resistant cast iron material in accordance with a preferred embodiment of the present invention, a cast iron alloy material comprising 2.8 weight % of C, 5.1 weight % of Si, 0.35 weight % of Mn, 0.042 weight % of P, 0.008 weight % of S, 0.8 weight % of Mo, 0.03 weight % or less of Ni, 0.3 weight % of V, 0.04 weight % of Cr, 0.02 weight % of Sb, and the balance of Fe, as shown in Table 1 below, was casted by a commonly used casting method. The resulting cast iron was annealed in a manner that the temperature was raised up to 960° C. at a rate of 20° C./min, the raised temperature was kept for 20 minutes and slowly cooled at a rate of 20° C./min down to 400° C. and then air cooled.

TABLE 1 Chemical constituents (Wt %) Examples C Si Mn P S Cr Ni Cu Mo V Sb Mg Example 2.80 5.10 0.35 0.042 0.008 0.04 0.03 — 0.80 0.30 0.02 — Comparative 3.31 4.16 0.32 0.048 0.03 0.02 0.03 — 0.65 — — 0.26 Example 1 Comparative 3.69 2.84 0.26 0.142 0.02 0.06 0.13 0.03 0.15 — — 0.034 Example 2

Comparative Example 1

A heat-resistant spheroidal graphite cast iron conventionally used as a material for an exhaust manifold was prepared by casting a cast iron alloy material comprising 3.31 wt % of C, 4.16 wt % of Si, 0.32 wt % of Mn, 0.048 wt % of P, 0.03 wt % of S, 0.65 wt % of Mo, 0.03 wt % of Ni, 0.02 wt % of Cr, 0.026 wt % of Mg, and the balance of Fe as shown in Table 1.

Comparative Example 2

Another heat-resistant spheroidal graphite cast iron conventionally used as a material for an exhaust manifold was prepared by casting a cast iron alloy material comprising 3.69 wt % of C, 2.84 wt % of Si, 0.26 wt % of Mn, 0.042 wt % of P, 0.02 wt % of S, 0.45 wt % of Mo, 0.13 wt % of Ni, 0.06 wt % of Cr, 0.034 wt % of Mg, and the balance of Fe as shown in Table 1 above.

Experimental Example 1 High-Temperature Tensile Strength

To compare the properties of the samples prepared in Example and Comparative Examples 1 and 2, high-temperature tensile tests were carried out by a commonly used equipment.

As shown in FIG. 1, the sample of the Example showed the highest high-temperature tensile strength at its surface temperature of 800° C. that is a severe condition of the exhaust system. The improved high-temperature strengths resulted from the solid solution strengthening effect with the addition of Mo and from the precipitation strengthening effect of VC with the addition of V. Moreover, it is confirmed that the combination the constituents added provides more increased effects of the high-temperature strength.

Experimental Example 2 High-Temperature Oxidation Resistance

To compare the properties of the samples prepared in the Example and Comparative Examples 1 and 2, high-temperature oxidation tests were carried out by a commonly used equipment.

As shown in FIG. 2, the sample of the Example showed the lowest oxidation reduction, which resulted from the generation of Fe₂SiO₄ that has a close connection with the interface between oxide layers to improve the oxidation resistance based on the Si content.

As described above, high-silicon ferritic heat-resistant cast irons of the present invention can provide improved high-temperature properties, high-temperature oxidation resistance of the product cast irons. Also, such cast irons can be manufactured by a cost-effective method.

The invention has been described in detail with reference to preferred embodiments thereof. However, it should be understood that various modifications and variations of the present invention may be made by those skilled in the art without departing from the spirit and the technical scope of the present invention as defined by the appended claims. 

1. A high-silicon ferritic heat-resistant cast iron having improved high-temperature strength and oxidation resistance comprising: 2.5˜3.8 weight % of C, 5.0˜7.0 weight % of Si, 0.2˜0.8 weight % of Mn, 0.05 weight % or less of P, 0.02 weight % of less of S, 0.5˜1.2 weight % of Mo, 0.1 weight % or less of Ni, 0.5 weight % or less of V, 0.01˜0.1 weight % of Cr, 0.05 weight % or less of Sb, and the balance of Fe.
 2. A method for preparing a high-silicon ferritic heat-resistant cast iron having an excellent high-temperature strength and a high oxidation resistance; comprising the steps of: (a) casting an iron alloy material comprising 2.5˜3.8 weight % of C, 5.0˜7.0 weight % of Si, 0.2˜0.8 weight % of Mn, 0.05 weight % or less of P, 0.02 weight % of less of S, 0.5˜1.2 weight % of Mo, 0.1 weight % or less of Ni, 0.5 weight % or less of V, 0.01˜0.1 weight % of Cr, 0.05 weight % or less of Sb, and the balance of Fe; and (b) annealing the cast iron in a manner that the temperature is raised up to 960° C. at a rate of 20° C./min, the raised temperature is kept for 20 minutes and them slowly cooled down to 400° C. at a rate of 20° C./min.
 3. A motor vehicle comprising the cast iron of claim
 1. 4. A motor vehicle comprising a part made by the method of claim
 2. 